Electroluminescent cell with integrated switching control

ABSTRACT

The controlled EL cell of this invention includes in addition to the elements of a conventional EL cell a resistive heater film which is connected between a trigger and ground electrode, and a thermally sensitive switching control layer. The exciting voltage of the EL cell is continuously applied across both the phosphor layer and the switching layer. In the absence of a trigger pulse, the switching layer is in a high resistance state and the voltage across just the phosphor layer is maintained below the threshold potential required for illumination. The switching layer is responsive to the heat generated by a trigger pulse through the resistive layer to switch to a low resistance state to thereby act as a ground plane for causing the full potential to be applied across the phosphor to effect illumination.

United States Patent 1191 Kabaservice et a1.

1 1 ELECTROLUMINESCENT CELL WITH INTEGRATED SWITCHING CONTROL [76] Inventors: Thomas P. Kabaservice, 305 E.

Roanake St., Blacksburg, Va. 24060; Francis E. Baker, Jr., 9937 Greenbelt Rd. No. 204, Lanham, Md. 20801 [22] Filed: Apr. 9, 1973 21 Appl. No.: 349,500

[52] US. Cl 315/ 71, 250/213 A, 313/108 A [51] Int. C1. H01j 7/44 [58] Field of Search 250/213 A; 338/23, 24;

14 1 Apr. 23, 1974 Primary Examiner-Walter Stolwein Attorney, Agent, or Firm-R. S. Sciascia; Q. E. Hodges [57] ABSTRACT The controlled EL cell of this invention includes in addition to the elements of a conventional EL cell a resistive heater film which is connected between a trigger and ground electrode, and a thermally sensitive switching control layer. The exciting voltage of the EL cell is continuously applied across both the phosphor layer and the switching layer. In the absence of a trigger pulse, the switching layer is in a high resistance state and the voltage across just the phosphor layer is maintained below the threshold potential required for illumination. The switching layer is responsive to the [56] References C ted heat generated by a trigger pulse through the resistive UNITED STATES PATENTS layer to switch to a low resistance state to thereby act 1,741,231 12 1929 Grondahl 338/24 as a gmund Plane causing the full POtential be 3,034,011 5/1962 Nisbet et a1" 250/213 X A applied across the phosphor to effect illumination.

3,132,276 5/1964 Yando 313/108 A 3,621,446 11/1971 Smith 338/23 5 Clams 3 Drawmg 9,TRANSPARENT TOP ELECTRODE //11, INSULATOR l N 5 U ATO R PHOSPHOR N 6, SWITCHING LAYER 4, GROUND 2 ,RESISTIVE LAYER ELECTROLUMINESCENT CELL WITH INTEGRATED SWITCHING CONTROL The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefore.

BACKGROUND OF THE INVENTION The invention relates generally to electroluminescent cells and specifically to a solid-state device capable of controlling whether an electroluminescent cell is illuminated or extinguished.

The invention is particularly applicable to large arrays of cells constituting a matrix type display and provides major advantages over conventional methods of control. In a conventional electroluminescent (hereinafter abbreviated EL) matrix display an EL cell is formed by sandwiching a high-field phosphor (e.g., zinc sulfide activated with manganese) between two insulating films. The'insulating films allow the application of an intense electric field across the phosphor film without thermal destruction of that material. Narrow strip electrodes are then applied to the top and bottom of the cell at right angles to each other, forming x and y lines. One set of electrodes is made transparent. An EL matrix element is defined by the intersection of an x and y line, and is illuminated by applying a large a.c. voltage (typically 200 v. rms. at KHz) between the lines. Switching this large exciting voltage from element to element constitutes a major problem in displays of this type. Ideally, switching of the exciting voltage from element to element would be controlled by large-scale integrated circuitry, in which form very complex logic circuits can be obtained at very low cost. Unfortunately, the exciting voltage is much too large to be handled directly by integrated logic. As a result, high-voltage driving circuits must be used between the switching logic and the EL matrix itself. These highvoltage devices are relatively expensive and bulky, and difficult to mate" with the display. A method of controlling an EL matrix that is completely integrated with the display and could be driven directly from integrated logic would be a major advance in the state of the art.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide an EL cell that can be driven directly from integrated control logic.

It is a further object of this invention to provide an EL cell wherein the integrated control logic is completely integrated with the cell.

Briefly stated, in accordance with the above objects, the controlled EL cell of this invention includes in addition to the elements of a conventional EL cell a resistive heater film which is connected between a trigger and ground electrode, and a thermally sensitive switching control layer. The exciting voltage of the EL cell is continuously applied across both the phosphor layer and the switching layer. In the absence of a trigger pulse, the switching layer is in a high resistance state and the voltage across just the phosphor layer is maintained below the threshold potential required for illumination. The switching layer is responsive to the heat generated by a trigger pulse through the resistive layer to switch to a low resistance state to thereby act as a ground plane for causing the full potential to be applied across the phosphor to effect illumination.

These and other objects and advantages will become apparent from the following detailed description taken in conjunction with the accompanying drawings, and the novel features will be particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I shows a cross sectional view of one embodiment of the EL matrix element of the invention.

FIG. 2 shows a cross sectional view of another embodiment of the EL matrix element of the invention.

FIG. 3 shows a schematic view of a matrix display utilizing the EL matrix element of the embodiment of FIG. 2.

DETAILED DESCRIPTION In the illustrated embodiment of FIG. 1 an insulating substrate 1 such as glass has applied to one surface thereof a resistive heating layer 2 which is connected between trigger electrode 3 and ground electrode 4. Switching layer 6 is in contacting relationship with resistive layer 2, and electrodes 3 and 4; although it is not necessary to be in contact with trigger electrode 3. The phosphor layer 7 is insulated on the bottom from switching layer 6 by dielectric insulating film 8, which may be SiO and similarly insulated on top from transparent electrode 9 by dielectric insulating layer 11.

Resistive heating layer 2 may be of a refractory metal such as nichrorne. Switching layer 6 could consist of one of the oxides of vanadium or titanium that display a large conductivity change at a given temperature, a switching chalcogenide glass, or any other material displaying a large negative temperature coefficient of resistivity. Phosphor layer 7 may be selected from any of the well known high field phosphors such as a suitably activated and coactivated phosphor of the zinc-cadmin sulfo-selenide family. Transparent top electrode 9 may be comprised of tin oxide or reduced titanium dioxide or any other suitable transparent conducting material.

A suitable thickness for the various layers is approximately 25 microns with the exception of the phosphor layer 7 which would normally be of a standard thickness of 2 microns. Of course, these thicknesses can be varied depending on the application requirements.

The fabrication of the EL cell may be effected by standard thin film techniques such as thin film deposition by evaporation, sputtering, or reactive sputtering.

In operation, a high-voltage a.c. signal (the exciting voltage of the EL cell) is continually applied between the top electrode 9 and ground electrode 4. In the absence of a trigger pulse, the switching layer is at ambient temperature and in a high-resistance state. Thus, the voltage must drop across not only the EL cell, but also the additional capacitance and resistance of the switching and resistive films. This additional series impedance keeps the voltage across the cell below the threshold for luminance. When a trigger pulse is applied, (from a source not shown) the resistive heater quickly raises the temperature of the switching layer, bringing it into a state of high conductivity. The switching l'ayer now acts essentially as a ground plane for the EL cell,.across which the entire a.c. exciting voltage is now impressed, resulting in luminance. The cell reverts to the dark state when the trigger pulse is removed, since the switching layer reverts to a high resistance state. The exciting a.c. voltage as noted above is typically 200 v. rms at 5 KHz. It should be understood that the trigger pulse must be at least as long as one-half cycle of the exciting voltage.

This simple and unique method for control of EL cells has many advantages over conventional methods. It is completely integrated with the display. The fabrication of the control device is compatible with processes used to make thin-film EL cells. Thus, production costs for displays embodying the invention are held low. The large a.c. exciting voltage is not switched directly, so high-voltage devices are not required in the switching logic circuitry. The electrical energy that must be supplied by the trigger pulse to accomplish switching is a function of the area to be heated, the temperature required, and the thermal characteristics of the structure. Since the dimensions of the controlled cell are small in the case of matrix elements in highresolution arrays, the required trigger pulse energy is low and therefore the current and voltage levels are low and can be supplied by integrated logic.

A number of modifications could be made of the structure of FIG. 1 without changing the principle of operation. For example, the order of depositions could be reversed, with the transparent electrode applied directly to a transparent substrate, other depositions following to form the structure of FIG. 1 upside down.

Another modification having practical advantages is shown in FIG. 2, wherein like reference numerals apply to like elements as in FIG. 1. This structure is quite similar to that of FIG. 1, except that the lower insulation and switching layers have been interchanged. Placing an insulator between the heater and switching layer prevents current from the trigger pulse from flowing through the latter after switching. This is desirable to avoid the formation of filaments in the switching layer 6 which could result in non-uniform illumination of the cell. The lower insulating layer 8 in this structure is shown extending over the trigger electrode 3. This permits the trigger electrodes to cross over the ground electrodes of the elements without shorting out. The ground electrode is made slightly thicker so as to be in contact with the switching layer.

The operation of the FIG. 2 embodiment is similar to that of FIG. 1 except that the switching layer 6 acts like a resistor in series with the phosphor layer instead of a capacitor as in FIG. .1.

FIG. 3 shows a schematic view of a matrix containing EL matrix elements with the integrated control of FIG. 2 so as to visualize the significance of the crossing over electrodes.

While the principal application of this invention appears to be the control of BL matrix displays, the control method could be applied to arrays of any highvoltage, low-current thin-film device. Since an EL cell behaves electrically as a capacitor, the invention could also be used to control the voltage or the charge on any capacitive device.

It will be understood that various changes in the details, materials, steps, and arrangments of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.

What is claimed is:

1. A solid state electroluminescent cell and switching control logic integrated therewith comprising:

a switching layer comprised of material having a negative temperature coefficient of resistivity;

a high-field phosphor layer on one side of said switching layer;

means for continuously applying an alternating electrical potential across said switching and said phosphor layers in electrical series relationship;

a resistive heating layer connected between a trigger electrode and a ground electrode, said resistive layer being on the other side of said switching layer and in thermal contact with said switching layer, said ground electrode being in physical contact with said switching layer;

wherein said switching layer at ambient temperature is in a high resistance state and the series potential across just said phosphor layer is below the threshold required for illumination, and whenever a trigger pulse is applied to said trigger electrode the resulting resistive heat generated by said resistive layer causes said switching layer to switch to a low resistance state thereby acting as a ground plane causing the full electrical potential to be applied across said phosphor layer to affect illumination.

2. The solid state device of claim 1 wherein said means for continuously applying an alternating electrical potential includes:

a transparent electrode layer separated from said phosphor layer by a first insulating film; and,

a source of a.c. voltage connected across said transparent electrode and said ground electrode.

3. The solid state device of claim 2 further including:

a second layer of insulating film separating said switching layer from said phosphor layer; and

wherein said resistive layer is in direct physical contact with said switching layer.

4. The solid state device of claim 2 further including: i 

1. A solid state electroluminescent cell and switching control logic integrated therewith comprising: a switching layer comprised of material having a negative temperature coefficient of resistivity; a high-field phosphor layer on one side of said switching layer; means for continuously applying an alternating electrical potential across said switching and said phosphor layers in electrical series relatIonship; a resistive heating layer connected between a trigger electrode and a ground electrode, said resistive layer being on the other side of said switching layer and in thermal contact with said switching layer, said ground electrode being in physical contact with said switching layer; wherein said switching layer at ambient temperature is in a high resistance state and the series potential across just said phosphor layer is below the threshold required for illumination, and whenever a trigger pulse is applied to said trigger electrode the resulting resistive heat generated by said resistive layer causes said switching layer to switch to a low resistance state thereby acting as a ground plane causing the full electrical potential to be applied across said phosphor layer to affect illumination.
 2. The solid state device of claim 1 wherein said means for continuously applying an alternating electrical potential includes: a transparent electrode layer separated from said phosphor layer by a first insulating film; and, a source of a.c. voltage connected across said transparent electrode and said ground electrode.
 3. The solid state device of claim 2 further including: a second layer of insulating film separating said switching layer from said phosphor layer; and wherein said resistive layer is in direct physical contact with said switching layer.
 4. The solid state device of claim 2 further including: a second layer of insulating film separating said resistive layer from said switching layer; and, wherein said switching layer is in direct physical contact with said phosphor layer.
 5. The solid state device of claim 4 wherein said second layer of insulating film also covers said trigger electrode. 